Giuliani Appears to Veer Off Script. A Furor Follows.

WASHINGTON — Rudolph W. Giuliani has been President Trump’s lawyer for only a few days, and already he is causing political explosions.

In one of his first interviews since joining Mr. Trump’s legal team, Mr. Giuliani appeared to briefly stun Sean Hannity of Fox News on Wednesday night by asserting that the president had reimbursed his personal lawyer for a $130,000 hush payment to a pornographic film actress — contradicting his client, the president.

“I want to clarify something,” Mr. Hannity said, offering a do-over to Mr. Giuliani, the former mayor of New York, failed presidential candidate and longtime friend of Mr. Trump’s. Instead of taking a mulligan, Mr. Giuliani repeated the assertion.

It was another high-profile, slightly off-kilter moment for Mr. Giuliani, who has lived a life full of them.

A veteran of the no-holds-barred world of New York tabloids that Mr. Trump also inhabited for decades, Mr. Giuliani was an early and vocal supporter of Mr. Trump’s presidential campaign. He was also a forceful critic of Hillary Clinton, a longtime political rival and Mr. Trump’s opponent in the 2016 presidential race.

For Mr. Giuliani, the embrace of Mr. Trump’s candidacy at times appeared to be a likely entree back into the limelight of national politics after a disastrous 2008 bid for the White House. An energetic speech he delivered in 2016 at the Republican National Convention, at which Mr. Trump was nominated, prompted talk of Mr. Giuliani’s being appointed secretary of state or, perhaps, of his leading the Department of Homeland Security.

Those aspirations never materialized. Mr. Giuliani was passed over for both jobs — not once, but twice for each position.

Still, Mr. Giuliani has never wavered in his support for his fellow New Yorker. And when Mr. Trump was seeking a new lawyer — a high-wattage TV combatant who would not wilt under the pressures of the news media or legal adversaries — Mr. Giuliani seemed a natural pick.

But Mr. Giuliani has historically been something of a loose cannon who is not used to having his words carefully managed. (Just ask his campaign handlers from 2008, who were constantly frustrated by his freestyling ways.)

So it may have surprised nobody when Mr. Giuliani’s comments on Wednesday night on Fox News set off a frenzy on Twitter. Almost instantly, people were reposting video of Mr. Trump on Air Force One last month saying he knew nothing about the payments.

Asked about his remarks after finishing the Fox News interview, Mr. Giuliani said that what might have seemed to some viewers — including, perhaps, Mr. Hannity — like a slip of the tongue, was actually a planned disclosure.

“That removes the campaign finance violation, and we have all the documentary proof for it,” Mr. Giuliani said, explaining that the president and his other lawyers were well aware of what Mr. Giuliani intended to say on the program. He insisted he had spoken with the president before and after the interview on Fox News. Mr. Giuliani also dismissed the social media furor. There was no cause for concern.

But it may take some time to determine if he was correct.

Late Wednesday night, Common Cause, a government watchdog group, said Mr. Giuliani’s remarks bolstered its lawsuit accusing the president and his campaign of breaking the law by failing to disclose a contribution to his campaign.

“Giuliani seemingly thought he was doing President Trump a favor — but instead made Trump’s legal problems much, much worse,” said Paul S. Ryan, the group’s vice president for policy and litigation.

Whether or not that proves true, Mr. Giuliani is likely to have an opinion about it. And a combative one, at that.